rogue apostrophe

Would you write ‘four day’s journey’ or ‘four days journey’?

I am having a tussle with a sub. I know it’s ‘Long Day’s Journey into Night’ but surely the journey doesn’t belong to the four days, so it should be ‘four days journey’ - and presumably ‘a four-day journey’ would be even better?

Actually, instead of "four day's journey" or "four days journey", I would write "four days' journey", the plural possessive. "Four days' journey" means a "journey of four days". The possessive phrase is turned around to eliminate the "of" (yes, journey does "belong" to days) and days is plural. Of course, you can use "four-day journey" as well. It's not better per se, just different.